The effect of physiographic and hydrologic complexities and their alterations on the distribution of obligate freshwater dolphins

Abstract Physiographic and hydrologic complexities play major role in determining the habitat suitability for river dolphins. However, dams and other water development structures alter hydrologic regimes that degrade habitat conditions. For the three extant species of obligate freshwater dolphins, namely: Amazon dolphin (Inia geoffrensis), Ganges dolphin (Platanista gangetica), and Indus dolphin (Platanista minor), the threat is high as dams and water‐based infrastructure dotted throughout their distribution range impact dolphin populations by restricting their movement. But there is also evidence of localized increase in dolphin population in certain segments of habitats affected by such hydrologic alterations. Hence, the impacts of hydrologic alterations on dolphin distribution are not as binary as it seems. We aimed to ascertain the role of hydrologic and physiographic complexities in determining the distribution of the dolphins in their geographic ranges using density plot analysis and also to understand how hydrologic alterations in the rivers affect their distribution using a combination of density plot analysis and review of literature. The influence of some of the study variables such as distance to confluence and sinuosity was similar across species—for instance, all three dolphin species preferred slightly sinuous river segments and habitats near confluences. However, varying influences across species were observed for some other variables such as river order and river discharge. We assessed 147 cases of impacts of hydrological alterations on dolphin distribution by categorizing the reported impacts in nine broad types out of which habitat fragmentation accounted for the majority of the impacts (35%) followed by habitat reduction (24%). These endangered species of freshwater megafauna will experience further intensified pressures as more large‐scale hydrologic modifications such as damming and diversion of rivers are underway. In this context, basin‐scale water‐based infrastructural development planning should take into consideration the salient ecological requirements of these species to ensure their long‐term survival.

as dams and water-based infrastructure dotted throughout their distribution range impact dolphin populations by restricting their movement. But there is also evidence of localized increase in dolphin population in certain segments of habitats affected by such hydrologic alterations. Hence, the impacts of hydrologic alterations on dolphin distribution are not as binary as it seems. We aimed to ascertain the role of hydrologic and physiographic complexities in determining the distribution of the dolphins in their geographic ranges using density plot analysis and also to understand how hydrologic alterations in the rivers affect their distribution using a combination of density plot analysis and review of literature. The influence of some of the study variables such as distance to confluence and sinuosity was similar across species-for instance, all three dolphin species preferred slightly sinuous river segments and habitats near confluences. However, varying influences across species were observed for some other variables such as river order and river discharge. We assessed 147 cases of impacts of hydrological alterations on dolphin distribution by categorizing the reported impacts in nine broad types out of which habitat fragmentation accounted for the majority of the impacts (35%) followed by habitat reduction (24%). These endangered species of freshwater megafauna will experience further intensified pressures as more largescale hydrologic modifications such as damming and diversion of rivers are underway.
In this context, basin-scale water-based infrastructural development planning should take into consideration the salient ecological requirements of these species to ensure their long-term survival.

| INTRODUC TI ON
Physiographic and hydrologic complexities play a major role in maintaining the ecological dynamics of rivers (Poff et al., 1997), making riverine habitats suitable for apex predators such as dolphins (Reeves & Leatherwood, 1994). River dolphins show a preference for specific hydraulic nodes such as confluences, meanders, and deep pools, which provide refuge from hydraulic forcing combined with higher biological activity (Smith, 1993;Smith et al., 2009;Smith & Mansur, 2012). Alterations in the hydrologic regime of rivers caused by dams, barrages (low-gated dams for water diversion rather than storage), and water abstraction for irrigation and power generation are known to impair riverine ecosystems globally by degrading habitat conditions (Aggarwal et al., 2020;Poff et al., 2010).
As hydrophysical parameters are major determinants of dolphin distribution and abundance, their modifications through interactive effects of flow regulation can potentially deteriorate dolphin habitat by decreasing the overall carrying capacity of the rivers (Huang et al., 2012;Paudel et al., 2020). The three extant species of obligate freshwater dolphin-Ganges dolphin (Platanista gangetica), Indus dolphin (Platanista minor), and Amazon dolphin (Inia geoffrensis)-are impacted by the threats relating to the modification of hydrologic regimes and degradation of riverine ecosystems. The fourth obligate freshwater dolphin-Yangtze dolphin (Lipotes vexillifer)-has been deemed possibly extinct (Smith et al., 2020). The stressors are becoming more prominent with an increasing number of flowmodifying structures and water-based infrastructure such as dams and barrages being developed and/or under planning throughout the distribution ranges of the dolphin species.
As river cetaceans including dolphins have similar habitat requirements and shared sensory and morphological characteristics (Campbell et al., 2022), the influence of some hydrophysiographic variables on dolphin occurrences may generally be similar across freshwater dolphin species. For example, confluences are preferred by Amazon dolphins and Ganges dolphins as they are associated with stable flows and fish aggregation (Martin & da Silva, 2004;Smith et al., 2009). Meanders tend to have higher nutrient content and also a prevalence of eddy countercurrents, which can disorient fishes, thus facilitating predation by dolphins (Guizada & Aliaga-Rossel, 2016;Mazumder et al., 2014). Deep pools are known to facilitate longer dives and hence yield the highest usable area for dolphins (Paudel et al., 2021). A combination of deep and shallow river segments also supports activities such as resting, feeding, and reproduction (Mosquera-Guerra et al., 2020;Sinha & Kannan, 2014).
Hydrologic alterations by flow-modifying structures including dams, barrages, canals, irrigation diversions, embankments, loop cutting, spur dikes, and/or activities such as dredging have been reported to generally impact freshwater dolphins as they change the hydrological complexities of the riverine ecosystems. For instance, hydrologic alterations have restricted the movement of dolphins by affecting longitudinal and lateral habitat connectivity, rendering them isolated into several subpopulations (Braulik & Smith, 2019;Smith et al., 2000). Straightening of river channels affects meanders (Sinha, 2000), and water abstraction can impact tributary flow and cause a reduction in floodplain connectivity (Kelkar et al., 2010).
While the decline in the freshwater dolphin population has been attributed to hydrologic alteration by water-based infrastructures such as dams, there is also clear evidence of increase in population of dolphins in certain segments of rivers affected by such hydraulic structures. For instance, the Bijnor-Narora stretch of the Upper Ganges, the Upper Ganges Ramsar Site, and the Katarniaghat Wildlife Sanctuary in Uttar Pradesh, India, have witnessed increases in Ganges dolphin population mostly due to increase in water depth in the rivers (Kreb et al., 2010;Sinha & Kannan, 2014). An increase in sinuosity in certain sections of the Ganges river after the construction of Bijnor barrage has led to additional hydraulic refuge for dolphins (Sonkar & Gaurav, 2020). Moreover, canals can also provide refuges for feeding and mating on a short-term basis and can increase the aggregate habitat while providing greater refuge during anthropological disturbance (Prajapati, 2021;Smith et al., 2001).
Besides, other structures such as groins and spur dikes result in the creation of artificial eddy countercurrent habitats (Smith, Aminul Haque, et al., 1998), which dolphins utilize for predation of fish.
Hence, the impacts of flow alterations in riverine habitats are not as binary as they seem to be for river dolphins.
With this background, we hypothesize that the dolphin distributions will show an overall similar pattern in relation to hydrologic and physiographic parameters across species of obligate freshwater dolphins, with an overall variable influence of hydrological alterations on distributions varying both in direction and in contribution with respect to species. We therefore aim to first ascertain the role of hydrologic and physiographic complexities of rivers in determining the distribution of the extant obligate freshwater dolphin species and then discuss the changes in dolphin distributions induced by alterations in flow regimes. To this end, we have carried out a review of the research conducted on this phenomenon, and we have analyzed dolphin distribution for the entire ranges of the three dolphin species with an emphasis on the probable impacts of existing and proposed dams.

K E Y W O R D S
Amazon dolphin, barrages, biodiversity, dams, Ganges dolphin, hydrologic alterations, hydrologic complexities, Indus dolphin, obligate freshwater dolphins, review

| ME THODS
To ascertain the effect of physiographic and hydrologic complexities as well as variables relating to hydrologic alteration, we performed density plot analysis of presence-only data of the species plotted along variables. We also carried out a review of literature to assess the impacts of dams and other water-based infrastructures on obligate freshwater dolphins. edu/). The observations withheld or with imprecision greater than 1000 m and stated records prior to 1995 were discarded from the GBIF data. The duplicates of the records were also discarded. This yielded 965 presence records of the Ganges dolphin, 403 presence records for the Indus dolphin, and 925 presence records for the Amazon dolphin. We then used these presence records for density plot analysis of the environmental variables that determine the distribution of the dolphins. Majority of these records were recent with more than 35% recorded after 2017.
HydroSHEDS provides a series of gridded datasets from where we obtained hydrologically conditioned Digital Elevation Model (DEM) and flow accumulation data. RiverATLAS/HydroSHEDS is a global compendium of hydro-environmental variables in vector format, which have been derived from existing datasets. The data on these variables are freely available as river network lines. It comprises 56 hydro-environmental variables such as natural discharge consisting of 281 individual attributes such as the variable natural discharge represented in three forms-annual average, annual mean, and annual maximum along with 14 attributes of HydroRivers such as river order (Linke et al., 2019).
We have used 14 variables as physiographic and hydrologic variables while two other variables-degree of regulation and irrigated area extent-represent alterations (Table 1). For use in density plot analysis, the variables must be in raster form. Hence, the variables which were not available in raster format were rasterized with the v.to.rast module in GRASS GIS 7.8.4 at 30 arc seconds spatial resolution. We resampled hydrologically conditioned DEM from 3 arc second resolution to 30 arc seconds to make it compatible with other environmental layers. As the study species are exclusively aquatic, we clipped all the environmental variable layers by river networks for use as input layers.
The physiographic and hydrologic variables used here are important factors determining the distribution of dolphins. Confluences are preferred as they enable stable flows and fish aggregation, F I G U R E 1 Occurrence records of three obligate freshwater dolphin species in their entire distribution ranges compiled from records reported by different researchers, personal collection, and online repositories. meanders enable prevalence of eddy countercurrents, and feature higher nutrient content while deep pools enable higher usable areas.
Whereas alterations in the form of degree of regulation and irrigated area extent restrict the movement of dolphins and render population isolation.
Depth along river reach was calculated by dividing river volume by river area both of which were obtained from RiverATLAS/ HydroSHEDS. For calculating sinuosity, we first built polylines from the river segments with v.build.polyline and then we split those segments with v.split module both using GRASS GIS with a maximum segment length of 5000 m. This segment length was also chosen by Smith et al. (2008) for determining sinuosity as habitat selection variables for freshwater cetaceans.
Finally, we calculated sinuosity using the v.to.db module. For the calculation of distance to confluence, we used line intersection to identify the location of confluences. We then employed TA B L E 1 Physiographic and hydrologic variables and variables representing alterations used in the study. Hydrologically conditioned digital elevation model DEM whose flow direction defines the expected flow of water over the terrain Extracted from HydroSHEDS (−185) -8527 m (at 3 arc second) Stream gradient The ratio of the elevation drop within the river reach (i.e., the difference between min. and max. elevation along the reach) and the length of the reach.
Extracted from RiverATLAS/ HydroSHEDS 0-20,0000 decimeter/km Classical ordering of river River order following classical ordering system such that order 1 corresponds to main channel, order 2 corresponds to tributaries that flow into order 1 and so on.

1-13
River flow corresponding to logarithmic size class River order using flow to distinguish logarithmic size classes such that order 1 corresponds to river reaches with long-term average discharge ≥100,000 m 3 /s; order 2 represents discharge ≥10,000 m 3 /s and so on.

1-10
Strahler order River order following Strahler ordering system that defines stream order on the basis of hierarchy of tributaries such that when streams of same order join the resulting stream has one higher order.

| Density plot analysis
To ascertain the physiographic and hydrologic complexities pertaining to dolphin distribution represented by presence locations of the species, we created density plots of species presence-only data and physiographic and hydrologic variables: discharge, flow accumulation, stream elevation, stream gradient, stream order, distance from confluence, river volume, river area, depth, sinuosity and inundation extent as well as variables that show the effect of hydrologic alterations: degree of regulation and irrigated area extent. We also performed two-sample z-test to test the statistical significance of the difference between the values of variables associated with dolphin occurrence locations and the overall values for the overall study area at 95% confidence interval.
The habitat preference of dolphins noted in earlier studies is based on observations, which are mainly qualitative in nature. For instance, the preference for confluences by dolphins has been noted by many studies (Bashir et al., 2010;Sinha & Kannan, 2014). This study provides a quantitative account of such habitat preferences.

| Data compilation on hydrological alterations
For assessing the impacts of dams and other water-based infrastruc- Overall, we collected data from 45 articles for our analysis in this review (Supporting Information Annex II).
The impacts of hydrological alterations noted in the literature were classified into nine broad categories: competition, creation of suitable habitat condition, decline in food availability, extirpation, fishery overlap, habitat fragmentation, habitat reduction, pollution, and range reduction (Table 2).
Each impact was classified as positive, negative, or mixed. Positive impacts for instance include groins and spur dikes constructed for closing an old distributary channel resulting in creation of artificial eddy countercurrents (Smith, Aminul Haque, et al., 1998). Negative impacts include impacts such as reduction in upstream occurrence limit (Choudhary et al., 2012), whereas a mixed impact includes impact states such as reservoir creation harboring large amounts of fish species but drastically reducing fish diversity affecting dolphins because of their wide feeding spectrum (Tavera et al., 2010).
The type of hydrological alterations such as dams, barrages, loop cutting, and the rivers and countries where these alterations occurred were also noted. The Ganges river was further classified as Upper Ganges (from the origin till Varanasi), Middle Ganges (the segment between Varanasi and Rajmahal), and Lower Ganges (the section downstream of Rajmahal). The impacts were also broadly classified as being direct or indirect. For example, irrigation diversions aggravating fishing impacts on dolphins were considered an indirect impact (Khanal et al., 2016), whereas reduced flow caused by a hydrologic alteration represents direct impact on dolphins by reducing space availability, water velocity, and depth (Braulik et al., 2014).
The impacts were also classified as observed, speculated, or predicted impacts. Observed impacts include impacts based on observation statements such as, lack of observation of the dolphin in the Mahakali river flowing through Nepal due to lack of enough water in the upstream of the barrage (Smith et al., 1994). Impacts worded as "likely," "could be," "could have been", for example, reported declines in fish diversity associated with flow regulation resulting in impact on river dolphin (Braulik et al., 2014) were classified as speculated impacts. Predicted impacts are the impacts for which construction of the hydrologic alteration has not been completed but for which impacts have been predicted, for example predicted exacerbation of mercury pollution due to flooding of surface area by Jirau Dam (Tavera et al., 2010).

| Impacts of hydrological alterations on dolphins
The Amazon dolphin was found to be distributed mostly in rivers with lower degrees of regulation while the Ganges dolphin and Indus dolphin showed occurrence in rivers with moderate to higher degrees of regulation, respectively ( Figure 2, Table 3). This is an indica- Decline in fish likely due to interference with spawning cycle of migratory fishes due to construction of dam in Kailashpuri (Smith, 1993) Decline in food availability

Negative Speculated Direct
Extirpation of dolphins from Mahakali river in Nepal  Extirpation Negative Observed Direct Irrigation diversion aggravating fishery impact in already impacted sites (Khanal et al., 2016) Fishery overlap Negative Observed Indirect Indus dolphins split into five subpopulations (Braulik, 2006 The reservoir created by dam will probably be used by Bolivian river dolphin harboring large amounts of fish species but drastically reducing diversity affecting dolphins because of their wide feeding spectrum (Tavera et al., 2010) Decline in food availability

Variables Amazon dolphin Ganges dolphin Indus dolphin
Annual average discharge (m 3 /sec) *** *** *** have mostly been related to habitat reduction. Fishery overlap due to the water development has also been noted for this species but not for other dolphins. Hydrologic alterations have led to increase in the Ganges dolphin populations in several segments mostly due to increase in water depth. In the case of the Indus dolphin, the impacts are mostly related to habitat fragmentation.
Positive impact has been associated with barrage on the Beas River enabling sustenance of a small Indus dolphin population (Momblanch et al., 2022).
Indirect impacts of hydrological alterations are also a matter of great concern as their cumulative impact further aggravates the precarious condition of the dolphin species facing multitude of threats.
These impacts often manifest as ecological traps-the dolphins choose habitats based on their evolutionarily determined responses to environmental cues but they land instead on risky situations jeopardizing their survival. Likewise, increase in pollution was noted to have a negative impact on both the Amazon dolphin and the Ganges dolphin. For example, the Amazon dolphin habitats have seen increasing levels of mercury mostly due to mining activities (Trujillo, Crespo, et al., 2010), while in the case of the Ganges dolphin, the pollution is mostly linked to suspended solids and organotin compounds (Sinha, 2000).

| Inference on habitat conditions observed in dolphin distribution areas
The occurrence of the Amazon dolphins mostly in third classical order of rivers and rivers with smaller average discharge (80% reported occurrence in 0-1000 m 3 /s) and smaller relative size (80% reported occurrence in 0-25 ha) indicates their distribution mostly in tributaries and smaller rivers. Among the Amazon dolphins, females are known to favor the flooded forest in order to meet the energy requirement for their young and for their safety (Martin & da Silva, 2004). Lateral hydrological connectivity that enables seasonal habitat use is considered a key determinant of their distribution. Hydrologic alterations such as those caused by dams that cutoff such a connectivity would pose a severe blow to the life cycle of the species.
The Ganges dolphins are mostly distributed in tributaries and moderate size rivers as shown by their occurrence in mostly third classical order of rivers, and rivers with small average discharge (80% reported occurrence in 0-500 m 3 /s) and moderate relative size (80% reported occurrence in 25-175 ha). The Ganges dolphins migrate to smaller tributaries during the monsoon season, which explains the second peak of distribution corresponding to rivers of seventh order in terms of logarithmic size order (Sinha & Kannan, 2014;Sinha & Sharma, 2003;Smith, 1993). Besides, the occurrence of the species in depression areas is in agreement with preference for deeper habitats near shallow depositional areas as reported by Kelkar (2008).
In contrast, the Indus dolphins are mostly distributed in mainstem mostly in one classical order of river and large size rivers (80% reported occurrence in 25-725 ha). The historical range of the Indus dolphin is fragmented into 17 river sections currently (Braulik et al., 2014). While their movement is not restricted to the mainstem of the Indus river and frequent movements to canals and other tributaries are observed, a large number of strandings occur when the canal gates are closed requiring rescue of the dolphins (Waqas et al., 2012).
As stated in our hypothesis we do see, similarity across the obligate freshwater dolphin species in the patterns of distribution in relation to hydrologic and physiographic parameters especially with variables such as river discharge, flow accumulation, distance to confluence, and sinuosity. However, we also observe variations across species in the influence of some other variables-for instance, in terms of Strahler river order, the Amazon dolphins prefer smaller order rivers, the Ganges dolphins prefer moderate order rivers while the Indus dolphins prefer higher order rivers.

| Impact of hydrologic alterations from dams and other structures constructed or proposed in the distribution range of dolphins
Positive impacts of hydrologic alterations on dolphin distribution relate to the creation of suitable habitat conditions due to increase in countercurrent habitats, maintenance of minimum depth, increase in the number of deep pools, increase in river sinuosity, and increase in aggregate habitat (Bashir et al., 2010;Momblanch et al., 2022;Prajapati, 2021;Samad et al., 2022;Sinha, 2000;Smith, Aminul Haque, et al., 1998Sonkar & Gaurav, 2020).  (Aggarwal et al., 2020).
Species such as the Amazon dolphin which are already under threat due to mercury pollution will be subject to heightened risk of biomagnification due to hydrologic alteration by dams. Damming for hydroelectricity generation in South America has been noted to favor mercury methylation in the reservoirs and is associated with biomagnification of mercury along trophic chains including a variety of fish in the reservoirs and downstream reaches of dams (Pestana et al., 2019). Hence, hydrological alterations will also aggravate indirect impacts to the dolphins from stressors such as water pollution.
When hydrological alterations are planned, the following considerations should be taken into account: • Closing off old distributary channels can provide benefits of creation of eddy countercurrents and increase in river depth (Smith, Aminul Haque, et al., 1998). Hence, the effect of these projects needs to be further explored for consideration as options for reviving dolphin habitats.
• The fishery impacts especially in water diversion zones and water abstraction reaches should be given serious attention as dolphins have to deal with dual impacts of reduced flows and declining prey availability in such conditions.
• Release of pollutants in the water, especially in water abstraction areas should be regulated as reduced flows decrease the dilution capacity of rivers . In addition, withdrawal of surface water during the low-flow and dry seasons should take into account the ecological requirements of large riverine species such as the dolphins.
• Embankments have already been shown to impact the Ganges dolphins through reduction in hydraulic complexity and lateral connectivity of river ecosystems, elimination of spawning habitat for fish, leading to extirpation of dolphins in some cases Smith et al., 2000). Embankments can be even more threatening for the Amazon dolphins, which are known to favor the flooded forest habitats (Martin & da Silva, 2004).
• Dam construction authorities in South America must factor in the impact of dams on the Amazon dolphins given their vulnerability to stressors particularly pollution (Trujillo, Portocarrero, et al., 2010). Environmental Impact Assessment (EIA) of water-based infrastructure projects should account for the enhanced risks of biomagnification of mercury pollution brought about by hydrologic alterations.
• Both the Amazon dolphins and the Ganges dolphins showed preference for the second and third-order rivers (classical river order). Hence, hydrologic alterations on such rivers should also be critically evaluated for potential detrimental impacts on these species.
• All three species of obligate freshwater dolphins were found in a wide range of stream gradient and river discharge values. Hence, basin-scale development planning in the distribution ranges of the dolphins must take into consideration the existing and potential impact on the dolphins. A Cumulative Impact Assessment (CIA) approach will be instrumental to predict and manage basin/ sub-basin scale changes in flow regimes especially when multiple infrastructures that modify water courses such as dams, water diversion for irrigation, interbasin water transfer projects are proposed for a river system.

| Methodological considerations and limitations
In spite of a comprehensive data compilation effort, the study is associated with some limitations. Some river segments such as the Sundarbans Delta are not finely represented. Hence, many parts of these segments lack values of environmental variables. Likewise, the water depth presented in the study are approximations of river depths based on global river discharge estimates and simple hydraulic geometry laws (Linke et al., 2019). These do not exhibit actual hydraulic extremes found in the rivers. For instance. at its deepest point, the Amazon river is estimated to be 100 m deep. But the maximum depth observed in this study is just over 25 m, which points to a lack of fine-scale hydrologic data.
Another limitation concerns the determination of the distance from nearest confluence for the cells in the rivers. The distance to confluence from a point that falls in the same raster cell as the confluence is calculated as 0. The value of 0 for distance to confluence should not be taken at face value as the locations of dolphins can be anywhere in the 30 arc second resolution (which corresponds to resolution of 1000 m × 1000 m) within the raster cell containing the confluence. To deal with this issue, we have classified distance to confluence less than 1000 m as "<1000" while all other values are continuous.
Although efforts were made to collect as much occurrence records as possible, there are areas where the dolphin occurrence records are lacking. For instance, records of the Ganges dolphin in the Ghaghara river, India, were not available to us. Besides, most of the occurrence records of the Amazon dolphins were obtained from tagged dolphins (Mosquera-Guerra et al., 2021). These represent concentrated records of the species in specific rivers, which tends to over-represent the range of variables in those segments.

| CON CLUS ION
The distribution of obligate freshwater dolphins is dependent on physiographic and hydrologic complexities of the rivers. Overall, it is observed that the Amazon dolphin is mostly distributed in tributaries and rivers with smaller average discharge, and smaller relative size. Lateral hydrological connectivity enables seasonal habitat use in the flooded forest habitats and this is considered a key determinant of its distribution. The Ganges dolphin is mostly distributed in tributaries and rivers with smaller average discharge and moderate relative size. Besides, the species had occurrence records in depression areas in the rivers. In contrast, the Indus dolphin is mostly

AUTH O R CO NTR I B UTI O N S
Anu Rai was involved in conceptualization, methodology, software, formal analysis, investigation, data curation, writing-original draft preparation, writing-review and editing, visualization, and project administration. Tawqir Bashir was conceptualization, methodology, investigation, data curation, resources, writing-original draft preparation, writing-review and editing, and supervision. Elio Guarionex Lagunes -Díaz was involved in methodology, software, validation, visualization, and writing-review and editing. Bibek Shrestha was involved in investigation, and writing-review and editing.

ACK N OWLED G M ENTS
We would like to thank Linke et al. (2019) for creating a data compendium on hydro-environmental variables and making it open access without which the research could not be conducted.

CO N FLI C T O F I NTE R E S T S TATE M E NT
No competing interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
The references of the occurrences record have been provided in Annex. The data used for analysis is available from HydroSHEDS website (https://www.hydro sheds.org/).